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The development of the BSAC standardized method of disc diffusion testing
1Department of Microbiology, City Hospital NHS Trust, Birmingham B18 7QH, UK. Jenny.Andrews@cityhospbham.wmids.nhs.uk
The Journal of Antimicrobial Chemotherapy
|June 23, 2001
Summary
A standardized disc susceptibility testing method was field-tested in 19 labs. The study found the method reliable for most common bacteria, with specific adjustments for gentamicin and cefuroxime, improving antimicrobial susceptibility testing.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
- Bacterial Pathogenesis
Background:
- The British Society for Antimicrobial Chemotherapy (BSAC) Working Party on Susceptibility Testing developed a standardized disc diffusion method.
- This method was validated through a field trial across 19 diagnostic laboratories in the UK and Ireland.
Purpose of the Study:
- To evaluate the performance and reliability of a standardized disc susceptibility testing method in routine diagnostic laboratories.
- To identify potential issues and areas for improvement in the interpretation of bacterial susceptibility to various antimicrobial agents.
Main Methods:
- The study utilized semi-defined media and a semi-confluent bacterial inoculum.
- Zones of inhibition were measured and correlated with BSAC-specified Minimum Inhibitory Concentration (MIC) breakpoints for susceptibility interpretation.
- Clinical isolates and reference strains (ATCC, NCTC) with known resistance mechanisms or susceptibility profiles were tested.
Main Results:
- The standardized method demonstrated high reliability for common Enterobacteriaceae (e.g., Escherichia coli, Proteus mirabilis, Klebsiella spp.), with minor issues noted for gentamicin and cefuroxime.
- False susceptible reporting occurred for gentamicin-resistant P. mirabilis (MIC 2 mg/L). Cefuroxime results were unreliable due to MIC distributions straddling breakpoints, leading to breakpoint amendments.
- No major issues were observed for Pseudomonas aeruginosa. Disc testing was unreliable for teicoplanin susceptibility in coagulase-negative staphylococci. Extended incubation (24h) improved glycopeptide resistance detection in enterococci. Inconsistencies with fastidious organisms were linked to inoculum errors.
Conclusions:
- The standardized disc susceptibility testing method is largely effective and reliable for routine use, particularly for common bacterial pathogens.
- Refinements in breakpoints for cefuroxime and attention to inoculum preparation for fastidious organisms are necessary.
- The field survey data provides valuable information for diagnostic laboratories to monitor daily testing performance using control strains.